The invention relates to a converter having at least one converter module, preferably however a plurality of converter modules. The invention furthermore relates to a method for operating such a converter.
Converters are employed in electrical drive engineering in order to supply an electrical load, in particular an electric motor, with electrical power from a (power) network. An example of a field of application for converters here is to supply the drive motor or the drive motors of an electric vehicle. The power supply in question in this case is the battery circuit—carrying DC current—of a traction battery of the vehicle. On the other hand, for stationary applications the primary side of the converter is for the most part connected to a single- or multi-phase AC power supply.
A converter normally comprises a primary circuit to be connected to the network and a secondary circuit to be connected to the load. The primary circuit and the secondary circuit are connected with one another by way of an intermediate (power) circuit, into which an intermediate circuit capacitance is connected. Depending on the type of the network to be connected on the primary side the primary circuit is embodied as a DC chopper controller or power converter (rectifier). The secondary circuit in question is normally an inverter in the form of a single- or multi-phase full bridge circuit.
The physical unit formed from the primary circuit, the secondary circuit and the intermediate circuit (with the intermediate circuit capacitance arranged therein) is also referred to in the following as “converter module”. In particular in motor vehicle drive engineering modular converters are employed in which a plurality of such converter modules is connected in series on the primary side. Such a modular converter is known for example from the publication Lukas Lambertz et al. “Modularer Hochfrequenzumrichter für Fahrzeugantriebe”, EMA 2010, 08.-09.09.2010, Aschaffenburg (“Modular high-frequency converter for vehicle drives”).
Electric motors and other loads supplied by converters frequently have a power factor which significantly undershoots the value One (100%). During the operation of such a load, a periodic reversal of direction of the present power flow regularly occurs in the secondary circuit of the converter module connected to the load. During each period of the output voltage delivered to the load the converter module thereby experiences a single or multiple switch between “power flow as a motor”, where electrical power is delivered from the intermediate circuit to the load, and “power flow as a generator”, where the load feeds electrical power back into the intermediate circuit.
If the electrical power fed back cannot be delivered, or cannot be delivered quickly enough, to the network it must be stored in the intermediate circuit capacitance. If the intermediate circuit capacitance is not dimensioned as sufficiently high this can lead to an impermissible elevation of the intermediate circuit voltage and thereby to overloading of or even damage to the converter. Generously dimensioned intermediate circuit capacitances do however have an adverse effect on the installation space requirement, the production costs and the weight of the converter.